The challenge of specifying an optimal antenna geometry that supports a broad range of wavelengths is generally afforded at the expense of antenna ringing, polarization offsets, parasitic side-lobe generation, radiation efficiency or any combination thereof. End-fire or flare-end ringing occurs when a signal bounces back-and-forth between the feed-point and the flare end of an antenna. This is a particularly prominent problem for ultra-wideband antennas such as that described in U.S. Pat. Nos. 3,369,245 and 3,984,838 and by Carrel in [1].
A primary challenge of antenna design is to mitigate the forgoing problems without distorting the rising edge of the transmitted pulse or destabilizing the ultra wideband impedance characteristics of the antenna. Prior art employed combinations of flair end lump loading and impedance tapering to suppress end-fire ringing at the cost of rising edge distortion and poor radiation efficiency; see [2], and U.S. Pat. No. 4,679,007.
The quest for broadband antennas that are capable of effectively transmitting impulse signals or multiple carrier waves has been ongoing for nearly a half-century and is documented through prior art and public disclosure including the dipole antenna, U.S. Pat. No. 4,125,840; resistive loaded and tapered antennas, [3] and U.S. Pat. Nos. 4,642,645 and 4,803,495; printed circuit board antennas, [4] and U.S. Pat. No. 4,758,843; side-lobe suppression antennas, U.S. Pat. No. 4,376,940; and lump loading for maximal energy transfer, U.S. Pat. No. 4,679,007.
Lump loading alone does not mitigate the problem of end-fire ringing during the first several cycles and consequently target detection applications are impeded at close range. Tapered antennas address the problem of close range target detection very effectively by distributing bands of impedance across the antenna to convert the ringing energy into heat. However, this payoff is afforded at the expense of a substantial drop in radiation efficiency and an accompanying requirement for more powerful transmitter hardware. Moreover, the discrete interface at each tapered band creates parasitic side-lobes and induces reflections near the feed point that distorts the rising edge of the transmitted pulse. This is a particularly prominent problem for target identification systems because the rising edge of the pulse is used to induce reflections that carry sufficient spectral bandwidth to characterize the target. These reflections are only useful if the transmitted signal has very low levels of distortion.
More recent work by Shlager, Smith and Maloney partially addressed the problem by applying a resistive taper to bowtie antennas [5]. The devices were implemented by constructing bow-tie antenna leaves from three sections of material that were comprised of varying conductivities that followed the tapering guidelines in [2]. A continued effort by Askildsen, Thompson, Whites, et al. in 2004 expanded the applicability of resistive tapering for high-performance ultra wide band bow-tie antennas in [4]. These efforts further revealed that resistive tapering reduces the return signal of an ultra wideband (UWB) signal pulse.
Several recent designs were patented to address the deficiencies of the above listed prior art including a low side-lobe resistive reflector antenna, U.S. Pat. No. 5,134,423; a low profile antenna, U.S. Pat. No. 5,184,143; a top loaded Bow-Tie antenna, U.S. Pat. No. 6,323,821; a closely coupled directive antenna, U.S. Pat. No. 6,025,811; a tapered, folded monopole antenna, U.S. Pat. No. 6,774,858. Each of these prior disclosures employed unique methods to mitigate known problems of the expired patents that were described earlier, yet none fully and simultaneously address the problems of end-fire ringing, consistent impedance characteristics, the rising edge distortion on the transmitted pulse, parasitic side lobe generation, non-uniform polarization artifacts, radiation efficiency, or any combination thereof.
While prior art does substantially improve select antenna parameters, these methods introduce new design tradeoffs that interfere with antenna performance. This invention applies a novel approach that leverages on the principles of shielded closed loop antennas [7], ultra wide band antenna design techniques, and impedance tapering to devise an impulse antenna that mitigates the foregoing. The invention simultaneously provides efficient canceling for balanced oppositely polarized signals and safe dissipation for unbalanced signal energy.